Light Guide Assembly for an LCD or the Like

ABSTRACT

An apparatus and method for assembling a display device to which is coupled a light guide that has smooth surfaces for light-transmission and at least one surface that is for light scattering and internal reflection. The method includes depositing a reflective coating on that surface, and installing the display device with coupled light guide in an interior of a meter housing or casing. A portion of the casing interior is filled with a component, such as epoxy, for sealing the case interior to protect the display device and light guide against water and dust penetration. All surfaces of the light guide are protected from direct contact with the epoxy. This approach greatly simplifies the assembly process and yields a robust display that also prevents light leakage from the light guide.

FIELD OF THE INVENTION

This invention relates to a backlit display device such as a liquidcrystal display (LCD), and in particular to light guide assembly thatprovides the back light for the display.

BACKGROUND AND SUMMARY OF THE INVENTION

Meters used on motorcycles include digital displays of running time(hours), engine temperature and other information. Since the motorcyclesmay be of the trail-type for traversing rough terrain, the meters mustbe robust to withstand vibration and shock. The meters must also be wellsealed to prevent penetration of fine dust and liquid into the meter.

The meters of interest here include, among other components, a digitalLCD that is backlit by light that is provided by a light guide. In thiscase, the light guide is a transparent glass-like solid through whichlight propagates from one end of the guide to another. The light guideis coupled to the display and transmits light from a source, such as oneor more light emitting diodes (LEDs) carried on a nearby printed circuitboard, to the back side of the display for illuminating or “backlighting” the display for viewing through its transparent front ordisplay side. The back lighting is especially helpful for viewing underlow ambient light conditions.

The meter components are housed in a casing that is mounted to themotorcycle. A cable is often included to deliver information from aremote sensor to the meter. An opening in the casing permits viewing ofthe front of the display. To enhance the robustness of the meter, theassembly process may include a liquid epoxy resin that is injected intothe casing interior after the display, light guide, and associatedcircuit board are in place. The epoxy cures to encapsulate thecomponents against penetration by damaging liquid or dust. The epoxyalso serves a dual purpose of anchoring the components in the casinginterior for securing the components in place, and for dampingvibrations that are received by the meter casing to thus protect theinterior components.

One problem with the epoxy encapsulation technique just mentioned isensuring that the epoxy does not come into direct contact with the lightguide, which may be, for example, formed of optically-clear acrylic.Such contact with the exterior of the light guide will change therefraction characteristics of the outer surface of the guide so thatlight inside the guide (and propagating from the source to back lightthe display) will be absorbed by the epoxy. This absorption reduces theillumination of the display to unacceptably low levels. The absorbed,lost light is sometimes referred to as light that has “leaked” from thelight guide.

One prior solution to the forgoing problem is to place a reflective tapearound the exterior of the surfaces that are intended to reflect lightalong the light guide. This approach, however, is time consuming andrequires a labor-intensive assembly process. Moreover, given the complexshape of some light guides it is difficult to effectively apply the tapeto all of the surface portions that may be exposed to contact with thelater applied epoxy.

The present invention is directed to a solution to the forgoing problemsand provides, among other things, an innovative way of assembling adisplay device to which is coupled a light guide that has smoothsurfaces for light-transmission and at least one surface that isconfigured for light scattering and internal reflection. The methodincludes depositing on that surface a reflective coating, and installingthe display device with coupled light guide in an interior of a meterhousing or casing. At least a portion of the casing interior is filledwith an epoxy for sealing the display device with coupled light guideagainst water and dust penetration. All surfaces of the light guide areprotected against direct contact with the epoxy or with any contaminant.This approach greatly simplifies the assembly process and yields arobust display without light leaking from the light guide.

Other advantages and features of the present invention will become clearupon study of the following portion of this specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary motorcycle meter thatincludes a display with which the light guide assembly of the presentinvention may be employed.

FIG. 2 is an enlarged cross section of a meter that includes a displaywith coupled light guide assembly of the present invention.

FIG. 3 is a perspective view of the back of a light guide component ofthe present invention.

FIG. 4 is an enlarged detail view of the cross section of FIG. 2 showingprotuberances in the light guide for scattering and reflecting lighttoward an outlet surface of the light guide.

FIG. 5 is a perspective view of the front of the light guide componentof the present invention.

FIG. 6 is a bottom plan view of the light guide component.

FIG. 7 is a perspective view like FIG. 5 but rotated 180 degrees about avertical axis.

DETAILED DESCRIPTION

Shown in FIG. 1 is an exemplary motorcycle meter 20 that includes adisplay with which the light guide assembly of the present invention maybe employed. The meter 20 houses internal components inside of a casing22, which can be two casing parts attached together. The casing includesan opening 23 on the side of the meter facing the viewer. The meter caninclude any of a variety of manual control buttons and knobs 26.

With reference to FIG. 2, a display device such as a liquid crystaldisplay (LCD) 28 is mounted in the interior space 30 of the casing withthe edges of the LCD abutting an opening 23 in the casing. A rectangulargasket 24 is present between the edges of the LCD 28 and the opening 23.Other components of the meter 20 are also carried in this space 30,including the meter's printed circuit board (PCB) 32 with circuitcomponents mounted thereto, as well as a power supply, which in thisinstance can be a coin cell 34.

One of the components mounted to the PCB 32 is an array of lightemitting diodes LEDs 36, which may comprise three separate diodes,emitting blue-, red- and green-wavelength light. In this embodiment,there are two such LED arrays 36 (hereafter referred to as simply“LEDs”) carried on the PCB.

The LCD 28 is backlit with light that is directed through a backlit face38 on the inward-facing surface of the LCD, so that the display face 40on the opposite surface of the LCD is sufficiently illuminated forviewing through the opening 23.

With reference to FIGS. 2-7, a light guide 42 formed in accordance withthe present invention is coupled to the LCD 28 and transmits light fromthe LEDs 36 on the PCB to the backlit face 38 of the LCD 28. Preferably,the light guide 42 is a transparent thermoplastic formed of PMMA(Poly(methyl methacrylate), the material is hereafter referred to asacrylic.

The light guide 42 includes a downwardly extending pair of inlets 44that each include a recess 45 (FIG. 3) for receiving and enclosingtherein an LED 36 that protrudes from the surface of the PCB 32. As aresult, light from the LEDs is directed into the core of the light guidefor propagating along its length.

The general path of the light propagating through the light guide 42 isdepicted by arrows 46 in FIGS. 2 and 4. Generally, the light moves fromthe inlets 44 until encountering an angled surface 48 (FIGS. 2 and 5)whence the light is reflected to travel through a generally plate-shapedpart 50 of the light guide 42 that, when mounted to the casing 22 (FIG.2) extends across the entire area of the backlit face 38 of the LCD 28.

As indicated by the path arrow 46 the light propagating through thelight guide 42 emanates from an outlet surface 52 of the light guide 42.That surface 52 is adjacent to but slightly spaced from the backlit face38 of the LCD owing to a peripheral rim 54 integrally formed in thelight guide 42 to extend upwardly (FIG. 2) and surround the outletsurface 52 like a four-sided frame.

An adhesive is applied to the flat, contact surface 56 of the rim 54 tobond that surface to the edges of the backlit face 38 of the LCD 28.This configuration provides a peripheral seal surrounding thespaced-apart junction of the light guide outlet surface 52 and thebacklit face 38 of the LCD, thereby preventing fluid from moving betweenthe backlit face and the outlet surface, which would degrade the qualityof the display.

With particular reference to FIGS. 3 and 4, the light guide 42 includesa rear surface 58 that is opposite and parallel to the light guideoutlet surface 52. The rear surface 58 includes an array of integrallyformed protuberances 60 dispersed across the rear surface to protrudeoutwardly therefrom. In the present embodiment, the protuberances 60 arecylindrically shaped and act as light traps to capture some of the lightthat traverses the area of the plate-shaped part 50 of the light guide.The light captured or impinging on the traps (protuberances 60) isscattered and reflected generally toward and through the outlet surface52 of the light guide and into the backlit face 38 of the LCD to thusserve as back light for the display as described above.

It is noteworthy here that as the distance from the source of light,LEDs 36, increases, (that is, right to left in FIGS. 2 and 4) thediameters of the cylindrical protuberances 60 increases to for aprogressively larger set of light traps to reflect correspondinglyincreasing amounts of the light toward the LCD 28. This has the effectof compensating for inevitable losses of light intensity as the lightpropagates through the guide, and the result is a more uniformlyilluminated display than would be the case if the protuberances were allsame diameter.

While the cylindrical-shaped, outwardly protruding protuberances 60 aredescribed as preferred as light traps mentioned above, it iscontemplated that other configurations of the light guide rear surface58 may be employed for accomplishing the light scattering as discussedabove. For instance, the rear surface need not be planar, and may beconfigured with inwardly protruding recesses or pockets of variousregular and irregular shapes. Thus, as used here, the term protuberancescan be considered as extending inwardly or outwardly, and have variousshapes.

As noted earlier, with all of the meter components assembled, includingthe light guide assembly and coupled display as just described. Theremaining interior space 30 of the casing (FIG. 2) is filled, as byinjection, with a seal component 62, which in this embodiment comprisesa liquid, curable epoxy. A portion of the seal component 62 is depictedin FIG. 2 as cross hatching, with the understanding that the sealcomponent 62 flows into and fills all of the voids in the interior space30 around the meter components inside the casing. The epoxy cures toencapsulate the components against penetration by potentially damagingliquid or dust. The epoxy also serves a dual purpose of anchoring thecomponents in the casing interior for securing the components in place,and damping vibrations that are received by the meter casing to thusprotect the interior components excessive vibration.

As noted earlier, any light propagating through the acrylic light guide42 can be lost (leaked) from the guide before reaching the guide'soutlet surface 52 if the seal component 62 described above comes indirect contact with any surface of the guide. Accordingly, prior toapplying the seal component 62, and for that matter prior to assemblingthe light guide and related components, many of the exterior surfaces ofthe light guide 42 are coated with reflective material so that thereflective material is applied directly to the light guide, between theguide and the later-applied seal component.

In a preferred embodiment the reflective material is a thin film ofaluminum applied, for example, by vacuum deposition. FIG. 5 depicts thelight guide 42 with the outlet surface 52 facing upwardly. It iscontemplated that this surface is protectively masked during theapplication of the reflective coating since that surface is to remaintransmissive to light. With reference to FIGS. 3 and 5, the exteriorsurfaces 64 of the part of the guide that defines the inlets 44 receivethe reflective coating, although the recesses 45 are masked to preventcoating the region where the light enters the guide from the source LEDsas discussed above.

The contact surfaces 56 (FIGS. 2 and 5) of the rim 54 of the light guideare also coated with the reflective material before being bonded to thedisplay as describe above. It is noteworthy here that of the four sidesof the rim 48, the reflective coating need not be applied to the contactsurfaces 56 corresponding to the sides that are generally parallel tothe path of the light across the guide (that is, the short sides in FIG.5) since loss of light at those edges would not noticeably degrade thelevel of illumination of the display. Nonetheless, these sides need notbe masked and can in fact be coated with reflective material withoutdeleterious effect on the display.

The reflective coating is also applied to the angled surface 48 (FIG.5), so that the reflective aspect of that surface, as discussed above,is maintained after the seal component 62 is added. Moreover, theexterior of the end surface 64 (FIGS. 2 and 3) opposite the angledsurface 48 has applied to it the reflective coating.

With particular reference to FIGS. 3, 4 and 6, the entire rear surface58 of the light guide has applied to it the reflective coating notedabove. FIG. 4 shows in cross section some of these protuberances 60 thatthat make up the array of integrally formed protuberances 60 dispersedacross the rear surface to extend therefrom. As noted earlier, thepreferred protuberances 60 are cylindrically shaped and thus defineassociated outer surfaces 66 (FIG. 4) that protrude for the otherwiseplanar portions 68 of rear surface 58. These surfaces 66, 68 provide theprimary function of the light traps as discussed above. The applicationof the thin film of reflective material, as by deposition of aluminum,ensures that all of these surfaces are coated to thus protect againstdirect contact of the surfaces with the later-applied seal component 62and attendant light leakage as discussed above.

By coating the entirety of the rear surface 58 (that is, all of surfaces66, 68) that surface is completely protected against potentially lightleaking contact with the seal component 62 as well as any othercontaminants (fine dust, etc.) that may over time make its way into theinterior of the meter casing.

Put another way, any surface of the light guide 42 that may be exposedto direct contact with the seal component when mounted in the casing 22may be covered with the reflective coating, but the surfacesspecifically mentioned as coated above should be so coated for bestresults.

While the foregoing description was made in the context of preferredembodiments, it is contemplated that modifications to those embodimentsmay be made without departure from the invention as claimed. Forinstance, backlit displays other than LCD type may be employed, and sealcomponents other than epoxy are contemplated.

1. A light guide assembly for coupling with a display device,comprising: a light-transmissive light guide having: an inlet forreceiving light that enters the light guide to propagate along a paththrough the light guide, an outlet surface from which emanates the lightthat propagates through the guide, the emanating light providing lightto a display device when the outlet surface is coupled to the displaydevice, a plurality of protuberances in a rear surface of the lightguide that is opposite the outlet surface of the light guide, theprotuberances having outer surfaces and being disposed in the path ofthe light that propagates within the guide so that propagating lightwithin the guide impinges upon and is scattered by the protuberances,and wherein the protuberances protrude from an otherwise planar portionof the rear surface; and a reflective coating applied to the outersurfaces of the protuberances and to the planar portion of the rearsurface such that light impinging on the protuberances and on the planarportion of the rear surface is reflected therefrom through the outletsurface of the light guide.
 2. The assembly of claim 1 wherein theprotuberances are three-dimensional members with associated outersurfaces and wherein the protuberances protrude outwardly from theplanar portion of the rear surface, the reflective coating being appliedto the associated outer surfaces of the protuberances as well as to theplanar portion of the rear surface.
 3. The assembly of claim 1 whereinthe reflective coating is applied by vacuum deposition.
 4. The assemblyof claim 1 further comprising: a display device coupled to the lightguide adjacent to the outlet surface; and a seal component adhered tothe reflective coating such that the coating is between the light guideand the seal component, the seal component surrounding and encapsulatingthe coupled light guide, but not contacting the outlet surface of thelight guide.
 5. The assembly of claim 4 wherein the display device has adisplay face and opposing backlit face and is coupled to the light guidewith the backlit face adjacent to the outlet surface of the light guide,the assembly further comprising a peripheral seal disposed around theoutlet surface between the light guide and display device for preventingfluid from moving between the backlit face and the outlet surface. 6.The assembly of claim 5 further comprising: a casing inside of which thecoupled display device and light guide are mounted so that the displayface is sealed against an opening in the casing, the casing having aninterior space behind the display device within which resides the lightguide; and wherein the seal component is an epoxy that fills a part ofthe interior space to encapsulate the coupled display device and lightguide therein.
 7. The assembly of claim 1 wherein the light guidefurther comprises at least one angled portion in the path of thepropagating light in the light guide, the angled portion having an outersurface to which the reflective coating is applied thereby to reflectthe propagating light.
 8. A method of assembling a display device towhich is coupled a light guide that has smooth surfaces forlight-transmission and at least one non-planar surface for lightscattering and internal reflection, comprising the steps of: depositingon the non-planar surface a reflective coating; installing the displaydevice with coupled light guide in an interior of a casing; and fillingat least a portion of the interior with an epoxy for sealing the displaydevice and coupled light guide against fluid penetration.
 9. The methodof claim 8 wherein the filling step is preceded with the step oflocating a peripheral seal between the coupled display device and lightguide for preventing the epoxy from reaching a light transmissivesurface of the light guide that is adjacent to the display device.
 10. Amethod of assembling a light guide for a display device, comprising thesteps of: providing a light-transmissive light guide having an inlet forreceiving light that enters the guide to propagate along a path throughthe light guide and an outlet surface from which emanates the light thatpropagates through the guide, the light guide also having a plurality ofprotuberances protruding from a rear surface of the guide that isopposite the outlet surface of the light guide, the protuberances havingouter surfaces and being disposed in the path of the light thatpropagates within the guide so that propagating light within the guideimpinges upon and is scattered by the protuberances, and wherein theprotuberances protrude from an otherwise planar portion of the rearsurface; and applying a reflective coating to the outer surfaces of theprotuberances and to the planar portion of the rear surface of the lightguide such that light impinging on the protuberances and on the planarportion of the rear surface is reflected therefrom through the outletsurface of the light guide.
 11. The method of claim 10 wherein theapplying step includes vacuum deposition of a thin film of reflectivematerial as the reflective coating.
 12. The method of claim 11 whereinthe reflective material comprises aluminum.
 13. The method of claim 10wherein the providing step includes forming the light guide fromacrylic.
 14. The method of claim 10 including the step of applying thereflective coating to the entire light guide except for the inlet andthe outlet surface thereof.
 15. The method of claim 10 furthercomprising the steps of: coupling to the outlet surface of the lightguide a display device; and adhering to the reflective coating a sealcomponent such that the coating is between the light guide and the sealcomponent, the seal component surrounding the coupled light guide awayfrom the outlet surface of the light guide.
 16. The method of claim 15wherein the adhering step comprises placing a curable liquid epoxyaround the light guide away from the outlet surface to serve as the sealcomponent upon curing of the epoxy.
 17. The method of claim 16 whereinin the step of placing the curable liquid epoxy is preceded with thestep of mounting the coupled light guide within the interior of acasing, and the placing step comprises injecting the liquid epoxy intothe interior of the casing.
 18. The method of claim 15 wherein thedisplay device has a display face and opposing backlit face and iscoupled to the light guide with the backlit face adjacent to the outletsurface of the light guide, the method further comprising the step oflocating a peripheral seal around the outlet surface and between thelight guide and display device for preventing fluid from moving betweenthe backlit face and the outlet surface.